Re: [Repeater-Builder] Re: Audio Reproduction

[scomind]

Hi Joe,   This is an interesting article: http://www.repeater-builder.com/rbtip/fmtheorydiscussion.html He suggests that de-emph came first to get around the rising noise of an FM receiver. In any case, I think most people agree that it was done for noise control purposes, which it does do well. If not, it would have been abandoned.   I disagree because there is no possible way to abandon it. The author says this: In the early days of FM, there were no varactor diodes so it was difficult, at best, to produce a direct FM modulator. PM modulation was, however, easy to achieve which is the reason that it was the "standard". I agree completely. So if PM is the standard, and it creates preemphasis on an FM demodulator, then you have no choice but to deemphasize if you want to recover flat audio. It's not like the designers ever had a choice.   Regarding links or repeaters with flat audio: I think that the only place the audio should be pre or de emped is in the end users radio, or in items that communicate as an end user (phone patches, voice synthesizers, etc.) There is no need to pre and de emp at a repeater, and especially no good reason for doing it on links.   I'm rather surprised, then, that you pointed out the article. At the end of the last paragraph, the author says what I have maintained for years:    I am not a believer in pulling receiver audio off the discriminator and directly into a "flat" transmitter. That originated in the ham circles and I don't believe that it will produce audio any better than "doing it right the first time"! Let the pre-emphasis and de-emphasis circuits do their job (it really results in a fairly flat response) and pay attention to the clipping levels. You'll have great audio and you will keep your deviation within spec, which is mandatory in today's FM bands. Actually, if we could get everyone to adjust their repeater deviation to 4 kHz, a lot of adjacent channel problems would be reduced to acceptable levels. Just a thought..    Thank you, Paul, K3VIX (a 26-year Motorola veteran designer).   73, Bob       Yahoo! Groups Links To visit your group on the web, go to: http://groups.yahoo.com/group/Repeater-Builder/   To unsubscribe from this group, send an email to: [EMAIL PROTECTED]   Your use of Yahoo! Groups is subject to the Yahoo! Terms of Service.